Multivalent metal ion battery having a cathode of recompressed graphite worms and manufacturing method

ABSTRACT

Provided is a multivalent metal-ion battery comprising an anode, a cathode, a porous separator electronically separating the anode and the cathode, and an electrolyte in ionic contact with the anode and the cathode to support reversible deposition and dissolution of a multivalent metal, selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof, at the anode, wherein the anode contains the multivalent metal or its alloy as an anode active material and the cathode comprises a cathode layer of an exfoliated graphite or carbon material recompressed to form an active layer that is oriented in such a manner that the active layer has a graphite edge plane in direct contact with the electrolyte and facing or contacting the separator.

FIELD OF THE INVENTION

The present invention relates generally to the field of rechargeable multivalent metal battery (e.g. zinc-, nickel-, magnesium-, or calcium-ion battery) and, more particularly, to a high-capacity cathode active layer of exfoliated graphite or carbon worms and a method of manufacturing the multivalent metal-ion battery.

BACKGROUND OF THE INVENTION

Historically, today's most favorite rechargeable energy storage devices—lithium-ion batteries—was actually evolved from rechargeable “lithium metal batteries” that use lithium (Li) metal as the anode and a Li intercalation compound (e.g. MoS₂) as the cathode. Li metal is an ideal anode material due to its light weight (the lightest metal), high electronegativity (−3.04 V vs. the standard hydrogen electrode), and high theoretical capacity (3,860 mAh/g). Based on these outstanding properties, lithium metal batteries were proposed 40 years ago as an ideal system for high energy-density applications.

Due to some safety concerns of pure lithium metal, graphite was implemented as an anode active material in place of the lithium metal to produce the current lithium-ion batteries. The past two decades have witnessed a continuous improvement in Li-ion batteries in terms of energy density, rate capability, and safety. However, the use of graphite-based anodes in Li-ion batteries has several significant drawbacks: low specific capacity (theoretical capacity of 372 mAh/g as opposed to 3,860 mAh/g for Li metal), long Li intercalation time (e.g. low solid-state diffusion coefficients of Li in and out of graphite and inorganic oxide particles) requiring long recharge times (e.g. 7 hours for electric vehicle batteries), inability to deliver high pulse power, and necessity to use pre-lithiated cathodes (e.g. lithium cobalt oxide, as opposed to cobalt oxide), thereby limiting the choice of available cathode materials. Further, these commonly used cathode active materials have a relatively low lithium diffusion coefficient (typically D˜10⁻¹⁶-10⁻¹¹ cm²/sec). These factors have contributed to one major shortcoming of today's Li-ion batteries—a moderate energy density (typically 150-220 Wh/kg_(cell)) but extremely low power density (typically <0.5 kW/kg).

Supercapacitors are being considered for electric vehicle (EV), renewable energy storage, and modern grid applications. The relatively high volumetric capacitance density of a supercapacitor (10 to 100 times greater than those of electrolytic capacitors) derives from using porous electrodes to create a large surface area conducive to the formation of diffuse double layer charges. This electric double layer capacitance (EDLC) is created naturally at the solid-electrolyte interface when voltage is imposed. This implies that the specific capacitance of a supercapacitor is directly proportional to the specific surface area of the electrode material, e.g. activated carbon. This surface area must be accessible by the electrolyte and the resulting interfacial zones must be sufficiently large to accommodate the EDLC charges.

This EDLC mechanism is based on surface ion adsorption. The required ions are pre-existing in a liquid electrolyte and do not come from the opposite electrode. In other words, the required ions to be deposited on the surface of a negative electrode (anode) active material (e.g., activated carbon particles) do not come from the positive electrode (cathode) side, and the required ions to be deposited on the surface of a cathode active material do not come from the anode side. When a supercapacitor is re-charged, local positive ions are deposited close to a surface of a negative electrode with their matting negative ions staying close side by side (typically via local molecular or ionic polarization of charges). At the other electrode, negative ions are deposited close to a surface of this positive electrode with the matting positive ions staying close side by side. Again, there is no exchange of ions between an anode active material and a cathode active material.

In some supercapacitors, the stored energy is further augmented by pseudo-capacitance effects due to some local electrochemical reactions (e.g., redox). In such a pseudo-capacitor, the ions involved in a redox pair also pre-exist in the same electrode. Again, there is no exchange of ions between the anode and the cathode.

Since the formation of EDLC does not involve a chemical reaction or an exchange of ions between the two opposite electrodes, the charge or discharge process of an EDL supercapacitor can be very fast, typically in seconds, resulting in a very high power density (typically 3-10 kW/Kg). Compared with batteries, supercapacitors offer a higher power density, require no maintenance, offer a much higher cycle-life, require a very simple charging circuit, and are generally much safer. Physical, rather than chemical, energy storage is the key reason for their safe operation and extraordinarily high cycle-life.

Despite the positive attributes of supercapacitors, there are several technological barriers to widespread implementation of supercapacitors for various industrial applications. For instance, supercapacitors possess very low energy densities when compared to batteries (e.g., 5-8 Wh/kg for commercial supercapacitors vs. 10-30 Wh/Kg for the lead acid battery and 50-100 Wh/kg for the NiMH battery). Modern lithium-ion batteries possess a much higher energy density, typically in the range of 150-220 Wh/kg, based on the cell weight.

In addition to lithium-ion cells, there are several other different types of batteries that are widely used in society: alkaline Zn/MnO₂, nickel metal hydride (Ni-MH), lead-acid (Pb acid), and nickel-cadmium (Ni—Cd) batteries. Since their invention in 1860, alkaline Zn/MnO₂ batteries have become a highly popular primary (non-rechargeable) battery. It is now known that the Zn/MnO₂ pair can constitute a rechargeable battery if an acidic salt electrolyte, instead of basic (alkaline) salt electrolyte, is utilized. However, the cycle life of alkaline manganese dioxide rechargeable batteries has been limited to typically 20-30 cycles due to irreversibility associated with MnO₂ upon deep discharge and formation of electrochemically inactive phases.

Additionally, formation of a haeterolite (ZnO:Mn₂O₃) phase during discharge, when Zn penetrates into the lattice structure of MnO₂, has made battery cycling irreversible. The Zn anode also has limitations on cycle life due to the redistribution of Zn active material and formation of dendrites during recharge, causing internal short-circuits. Attempts to solve some of these issues have been made by Oh, et al. [S. M. Oh, and S. H. Kim, “Aqueous Zinc Sulfate (II) Rechargeable Cell Containing Manganese (II) Salt and Carbon Powder,” U.S. Pat. No. 6,187,475, Feb. 13, 2001] and by Kang, et al. [F. Kang, et al. “Rechargeable Zinc Ion Battery”, U.S. Pat. No. 8,663,844, Mar. 4, 2014]. However, long-term cycling stability and power density issues remain to be resolved. Due to these reasons, the commercialization of this battery has been limited.

Xu, et al. US Pub. No. 2016/0372795 (Dec. 22, 2016) and US Pub. No. 2015/0255792 (Sep. 10, 2015) reported Ni-ion and Zn-ion cells, respectively, which both make use of graphene sheets or carbon nanotubes (CNTs) as the cathode active material. Although these two patent applications claim an abnormally high specific capacity of 789-2500 mAh/g based on the cathode active material weight, there are several serious problems associated with these two cells:

-   (1) There is no plateau portion in the charge or discharge curves     (voltage vs. time or voltage vs. specific capacity), unlike typical     lithium-ion batteries. This lack of a voltage curve plateau means     the output voltage being non-constant (varying too much) and would     require a complicated voltage regulation algorithm to maintain the     cell output voltage at a constant level. -   (2) Actually, the discharge curve for the Ni-ion cell exhibits an     extremely sharp drop in voltage from 1.5 volts to below 0.6 volts as     soon as the discharge process begins and, during most of the     discharge process, the cell output is below 0.6 volts, which is not     very useful. As a point of reference, the alkaline cell (a primary     battery) provides an output voltage of 1.5 volts. -   (3) The discharge curves are characteristic of surface adsorption or     electroplating mechanisms at the cathode, as opposed to ion     intercalation. Further, it appears that the main event that occurs     at the cathode during the battery discharge is electroplating. The     high specific capacity values reported by Xu, et al. are simply a     reflection on the high amount of Ni or Zn metal electroplated on the     surfaces of graphene or CNTs. Since there is an excess amount of Ni     or Zn in the anode, the amount of electroplated metal increases as     the discharge time increases. Unfortunately, the electrochemical     potential difference between the anode and the cathode continues to     decrease since the difference in the metal amount between the anode     and the cathode continues to decrease (more Zn or Ni is dissolved     from the anode and gets electroplated on cathode surfaces). This is     likely why the cell output voltage continues to decrease. The cell     voltage output would be essentially zero when the amounts of metal     at the two electrodes are substantially equivalent or identical.     Another implication of this electroplating mechanism is the notion     that the total amount of the metal that can be deposited on the     massive surfaces at the cathode is dictated by the amount of the     metal implemented at the anode when the cell is made. The high     specific capacity (as high as 2,500 mAh/g) of graphene sheets at the     cathode simply reflects the excessively high amount of Zn provided     in the anode. There is no other reason or mechanism for why graphene     or CNTs could “store” so much metal. The abnormally high specific     capacity values as reported by Xu, et al. were artificially obtained     based on the high amounts of Ni or Zn electroplated on cathode     material surfaces, which unfortunately occurred at very low voltage     values and are of little utility value.

Clearly, an urgent need exists for new cathode materials that provide proper discharge voltage profiles (having a high average voltage and/or a high plateau voltage during discharge), high specific capacity at both high and low charge/discharge rates (not just at a low rate), and long cycle-life for a multivalent metal secondary battery. Hopefully, the resulting battery can deliver some positive attributes of a supercapacitor (e.g. long cycle life and high power density) and some positive features of a lithium-ion battery (e.g. moderate energy density). These are the main objectives of the instant invention.

SUMMARY OF THE INVENTION

The invention provides a multivalent metal-ion battery comprising an anode, a cathode, and an electrolyte in ionic contact with the anode and the cathode to support reversible deposition and dissolution of a multivalent metal (selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof) at the anode, wherein the anode contains the multivalent metal or its alloy as an anode active material and the cathode comprises a cathode active layer of exfoliated graphite or carbon material having inter-flake pores from 2 nm to 10 μm in pore size. The multivalent metal alloy preferably contains at least 80% by weight of the multivalent element in the alloy (more preferably at least 90% by weight). There is no restriction on the type of alloying elements that can be chosen.

The exfoliated carbon or graphite material in the cathode active layer may be selected from a thermally exfoliated product of meso-phase pitch, meso-phase carbon, meso carbon micro-beads (MCMB), coke particles, expanded graphite flakes, artificial graphite particles, natural graphite particles, highly oriented pyrolytic graphite, soft carbon particles, hard carbon particles, multi-walled carbon nanotubes, carbon nano-fibers, carbon fibers, graphite nano-fibers, graphite fibers, carbonized polymer fibers, or a combination thereof. These carbon/graphite materials can be subjected to an expansion treatment (e.g. intercalation, oxidation, and/or fluorination), followed by thermal exfoliation to obtain exfoliated graphite/carbon worms.

The exfoliated graphite/carbon worms are preferably recompressed to form a cathode active layer of recompressed exfoliated graphite or carbon material that is oriented in such a manner that the layer has a graphite edge plane in direct contact with the electrolyte (to readily admit ions from the electrolyte and release ions into electrolyte) and facing or touching the separator (so that the ions permeating through the porous separator can readily enter the inter-flake spaces near the edge plane).

In certain embodiments, the layer of recompressed exfoliated graphite or carbon material has a physical density from 0.5 to 1.8 g/cm³ and has meso-scaled pores having a pore size from 2 nm to 50 nm. In some preferred embodiments, the layer of recompressed exfoliated graphite or carbon material has a physical density from 1.1 to 1.8 g/cm³ and has pores having a pore size from 2 nm to 5 nm. In certain embodiments, the exfoliated graphite or carbon material has a specific surface area from 10 to 1,500 m²/g. The specific surface area of the recompressed worms is typically from 10 to 1,000 m²/g, but more typically and preferably from 20 to 300 m²/g.

We have observed that a select multivalent metal (e.g. Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Ga, In, or Cr), when coupled with a presently invented layer of exfoliated graphite or carbon material (preferably recompressed), can exhibit a discharge curve plateau at approximately 1.0 volt or higher (up to 3.7 volts). This plateau regime of a discharge voltage vs. time (or capacity) curve enables the battery cell to provide a useful constant voltage output. A voltage output significantly lower than 1 volt is generally considered undesirable. The specific capacity corresponding to this plateau regime is typically from approximately 100 mAh/g to above 600 mAh/g.

This multivalent metal-ion battery can further comprise an anode current collector supporting the aluminum metal or aluminum metal alloy or further comprise a cathode current collector supporting the cathode active layer. The current collector can be a mat, paper, fabric, foil, or foam that is composed of conducting nano-filaments, such as graphene sheets, carbon nanotubes, carbon nano-fibers, carbon fibers, graphite nano-fibers, graphite fibers, carbonized polymer fibers, or a combination thereof, which form a 3D network of electron-conducting pathways. The high surface areas of such an anode current collector not only facilitate fast and uniform dissolution and deposition of metal ions, but also act to reduce the exchange current density and, thus, the tendency to form metal dendrites that otherwise could cause internal shorting.

The carbon or graphite material, such as meso-phase pitch, meso-phase carbon, meso carbon micro-beads (MCMB), coke particles, expanded graphite flakes, artificial graphite particles, natural graphite particles, highly oriented pyrolytic graphite, soft carbon particles, hard carbon particles, multi-walled carbon nanotubes, carbon nano-fibers, carbon fibers, graphite nano-fibers, graphite fibers, and carbonized polymer fibers, has an original inter-planar spacing d₀₀₂ from 0.27 nm to 0.42 nm prior to a chemical or physical expansion treatment and the inter-planar spacing d₀₀₂. is increased to from 0.43 nm to 2.0 nm after the expansion treatment.

The expansion treatment includes an expansion treatment includes an oxidation, fluorination, bromination, chlorination, nitrogenation, intercalation, combined oxidation-intercalation, combined fluorination-intercalation, combined bromination-intercalation, combined chlorination-intercalation, or combined nitrogenation-intercalation of the graphite or carbon material. This expansion treatment is followed by thermal exfoliation and recompression. Due to the expansion treatments, the carbon or graphite material can contain a non-carbon element selected from oxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, or boron.

Unconstrained thermal exfoliation typically results in exfoliated graphite/carbon worms that have inter-flake pores having an average size from 20 nm to 50 μm (more typically from 100 nm to 10 μm). The exfoliated graphite/carbon worms are then compressed to produce a layer or block of recompressed exfoliated carbon or graphite material that is oriented in such a manner that the layer has a graphite edge plane in direct contact with the electrolyte and facing or contacting the separator. The recompressed exfoliated carbon or graphite material typically has a physical density from 0.5 to 1.8 g/cm³ and has meso-scaled pores having a pore size from 2 nm to 50 nm.

The electrolyte may be selected from an aqueous electrolyte, organic electrolyte, polymer electrolyte, molten salt electrolyte, ionic liquid electrolyte, or a combination thereof. In the invented multivalent metal-ion battery, the electrolyte may contain NiSO₄, ZnSO₄, MgSO₄, CaSO₄, BaSO₄, FeSO₄, MnSO₄, CoSO₄, VSO₄, TaSO₄, CrSO₄, CdSO₄, GaSO₄, Zr(SO₄)₂, Nb₂(SO₄)₃, La₂(SO₄)₃, BeCl₂, BaCl₂, MgCl₂, AlCl₃, Be(ClO₄)₂, Ca(ClO₄)₂, Mg(ClO₄)₂, Mg(BF₄)₂, Ca(BF₄)₂, Be(BF₄)₂, tri(3,5-dimethylphenyl borane, tris(pentafluorophenyl)borane, Alkyl Grignard reagents, magnesium dibutyldiphenyl, Mg(BPh2Bu2)₂, magnesium tributylphenyl Mg(BPhBu3)2), or a combination thereof.

In certain embodiments of the present invention, the electrolyte comprises at least a metal ion salt selected from a transition metal sulphate, transition metal phosphate, transition metal nitrate, transition metal acetate, transition metal carboxylate, transition metal chloride, transition metal bromide, transition metal perchlorate, transition metal hexafluorophosphate, transition metal borofluoride, transition metal hexafluoroarsenide, or a combination thereof.

In certain embodiments, the electrolyte comprises at least a metal ion salt selected from a metal sulphate, phosphate, nitrate, acetate, carboxylate, chloride, bromide, or perchlorate of zinc, aluminum, titanium, magnesium, beryllium, calcium, manganese, cobalt, nickel, iron, vanadium, tantalum, gallium, chromium, cadmium, niobium, zirconium, lanthanum, or a combination thereof.

In the multivalent metal-ion battery, the electrolyte comprises an organic solvent selected from ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), methyl butyrate (MB), ethyl propionate, methyl propionate, propylene carbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), tetrahydrofuran (THF), toluene, xylene, methyl acetate (MA), or a combination thereof.

In certain embodiments, the layer of exfoliated carbon or graphite material also operates as a cathode current collector to collect electrons during a discharge of the battery and wherein the battery contains no separate or additional cathode current collector.

The cathode active layer of exfoliated graphite/carbon may further comprise a binder material (preferably an electrically conductive binder material), which bonds exfoliated graphite flakes together to form a cathode electrode layer. The electrically conductive binder material may be selected from coal tar pitch, petroleum pitch, meso-phase pitch, a conducting polymer, a polymeric carbon, or a derivative thereof. Non-conducting materials (e.g. PVDF, PTFE, SBR, etc.) may also be used.

Typically, the invented secondary battery has an average discharge voltage typically no less than 1 volt (more typically and preferably >1.5 volts) and a cathode specific capacity greater than 200 mAh/g (preferably and more typically >300 mAh/g, more preferably >400 mAh/g, and most preferably >500 mAh/g) based on a total cathode active layer weight. Some cells deliver a specific capacity >600 mAh/g.

Preferably, the secondary battery has an average discharge voltage no less than 2.0 volts (preferably >2.5 volts, more preferably >3.0 volts, and most preferably >3.5 volts) and a cathode specific capacity greater than 100 mAh/g based on a total cathode active layer weight (preferably and more typically >200 mAh/g, more preferably >300 mAh/g, and most preferably >500 mAh/g).

The present invention also provides a method of manufacturing a multivalent metal-ion battery. The method comprises: (a) providing an anode containing a multivalent metal (selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof) or its alloy; (b) providing a cathode comprising a layer of recompressed exfoliated carbon or graphite material (recompressed graphite/carbon worms or recompressed expanded graphite flakes, not graphene sheets); and (c) providing an electrolyte capable of supporting reversible deposition and dissolution of the multivalent metal at the anode and reversible adsorption/desorption and/or intercalation/de-intercalation of ions at the cathode; wherein the layer of recompressed exfoliated carbon or graphite material is oriented in such a manner that the layer has a graphite edge plane in direct contact with the electrolyte and facing or contacting a plane of the separator. Typically, this graphite edge plane is substantially parallel to the porous separator and, thus, can readily admit and accommodate ions that migrate through the separator. Preferably, the electrolyte contains an aqueous electrolyte, an organic electrolyte, a molten salt electrolyte, or an ionic liquid.

The method can further include providing a porous network of electrically conductive nano-filaments to support the multivalent metal or its alloy at the anode.

In the method, the step of providing a cathode preferably contains subjecting a carbon or graphite material to an expansion treatment selected from an oxidation, fluorination, bromination, chlorination, nitrogenation, intercalation, combined oxidation-intercalation, combined fluorination-intercalation, combined bromination-intercalation, combined chlorination-intercalation, or combined nitrogenation-intercalation, followed by thermal exfoliation at a temperature from 100° C. to 2,500° C.

In certain preferred embodiments, the procedure of providing the cathode includes compressing exfoliated graphite or carbon using a wet compression or dry compression to align constituent graphite flakes of the exfoliated graphite or carbon. The procedure can produce a layer or block of graphitic structure having a flake edge plane being parallel to the separator, enabling direct entry of ions from separator pores into inter-flake spaces with minimal resistance. Preferably, the procedure of providing the cathode includes compressing exfoliated graphite or carbon using a wet compression to align constituent graphite flakes of the exfoliated graphite or carbon, wherein wet compression includes compressing or pressing a suspension of exfoliated graphite or carbon dispersed in a liquid electrolyte intended for use in a multivalent metal-ion cell. Any of the aforementioned electrolytes can be utilized in this suspension. The electrolyte later becomes part of the electrolyte of the intended multivalent metal-ion battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) Schematic drawing illustrating the processes for producing intercalated and/or oxidized graphite, subsequently exfoliated graphite worms, and conventional paper, mat, film, and membrane of simply aggregated graphite or graphene flakes/platelets;

FIG. 1(B) An SEM image of exfoliated carbon (exfoliated carbon worms);

FIG. 1(C) An SEM image of graphite worms;

FIG. 1(D) Schematic drawing illustrating the approaches of producing thermally exfoliated graphite structures.

FIG. 1(E) A continuous process of producing recompressed exfoliated graphite, including feeding dry exfoliated graphite worms into the gap between a pair of two counter-rotating rollers or the gaps between several pairs of rollers.

FIG. 1(F) A schematic drawing to illustrate an example of a compressing and consolidating operation (using a mold cavity cell 302 equipped with a piston or ram 308) for forming a layer of highly compacted and oriented graphite flakes.

FIG. 1(G) Schematic drawing to illustrate another example of a compressing and consolidating operation (using a mold cavity cell equipped with a piston or ram) for forming a layer of highly compacted and oriented graphite flakes.

FIG. 2(A) Schematic of a multivalent metal secondary battery, wherein the anode layer is a thin multivalent metal coating or foil and the cathode active material layer contains a layer of thermally exfoliated graphite/carbon; and

FIG. 2(B) Schematic of a multivalent metal secondary battery cell, wherein the anode layer is a thin multivalent metal coating or foil and the cathode active material layer is composed of thermally exfoliated graphite/carbon, an optional conductive additive (not shown) and an optional binder (not shown).

FIG. 3(A) The discharge curves of three Zn foil anode-based Zn-ion cells; first one containing a cathode layer of original graphite particles, second one containing a cathode layer of thermally exfoliated graphite (with only light recompression; pore size from 20 nm to 3 μm), and third one containing a cathode layer of thermally exfoliated graphite (with heavy recompression; pore size range from approximately 2 nm to 20 nm; having an edge plane being parallel to the separator and in ion-contact with the separator).

FIG. 3(B) The discharge curves of two Ca-ion cells; one containing a cathode layer of thermally exfoliated graphite (with heavy recompression) having an edge plane being parallel to the separator and in ionic contact with the separator); the other having a graphite flake surface plane being parallel to the separator and in contact with the separator. The cell containing a cathode of original artificial graphite particles exhibits a very short discharge curve (<30 mAh/g); not shown in the chart.

FIG. 4 The discharge curves of two Ni mesh anode-based cells; one containing a cathode layer of original MCMB particles and the other a cathode layer of thermally exfoliated MCMB worms.

FIG. 5 The specific capacity of a Zn-MCMB cell (containing a cathode layer of recompressed exfoliated MCMB) and a V-CNF (containing a cathode of vapor grown carbon nanofibers exfoliated and recompressed), both plotted as a function of the number of charge/discharge cycles.

FIG. 6 The specific capacity of a Mg-ion cell containing a cathode layer of heavily recompressed exfoliated MCMBs and favorably oriented; and the specific capacity of a Mg-ion cell containing a cathode of lightly recompressed exfoliated MCMBs, both plotted as a function of the number of charge/discharge cycles. The electrolyte used was 1 M of MgCl₂:AlCl₃ (2:1) in monoglyme.

FIG. 7 Ragone plots of Mg-ion cells (electrolyte=1 M of tris(pentafluorophenyl)borane in tetrahydrofuran, 25° C.) and Ca-ion cells (electrolyte=0.45 M Ca(BF₄)₂ in EC:PC at 80° C.); the anode being Mg or Ca metal supported or unsupported by CNTs and graphene foam, respectively and the cathode being recompressed exfoliated artificial graphite.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As schematically illustrated in the upper portion of FIG. 1(A), bulk natural graphite is a 3-D graphitic material with each graphite particle being composed of multiple grains (a grain being a graphite single crystal or crystallite) with grain boundaries (amorphous or defect zones) demarcating neighboring graphite single crystals. Each grain is composed of multiple graphene planes that are oriented parallel to one another. A graphene plane or hexagonal carbon atom plane in a graphite crystallite is composed of carbon atoms occupying a two-dimensional, hexagonal lattice. In a given grain or single crystal, the graphene planes are stacked and bonded via van der Waal forces in the crystallographic c-direction (perpendicular to the graphene plane or basal plane). The inter-graphene plane spacing in a natural graphite material is approximately 0.3354 nm.

Artificial graphite materials also contain constituent graphene planes, but they have an inter-graphene planar spacing, d₀₀₂, typically from 0.32 nm to 0.36 nm (more typically from 0.3339 to 0.3465 nm), as measured by X-ray diffraction. Many carbon or quasi-graphite materials also contain graphite crystals (also referred to as graphite crystallites, domains, or crystal grains) that are each composed of stacked graphene planes. These include meso-carbon mocro-beads (MCMBs), meso-phase carbon, soft carbon, hard carbon, coke (e.g. needle coke), carbon or graphite fibers (including vapor-grown carbon nano-fibers or graphite nano-fibers), and multi-walled carbon nanotubes (MW-CNT). The spacing between two graphene rings or walls in a MW-CNT is approximately 0.27 to 0.42 nm. The most common spacing values in MW-CNTs are in the range of 0.32-0.35 nm, which do not strongly depend on the synthesis method.

It may be noted that the “soft carbon” refers to a carbon material containing graphite domains wherein the orientation of the hexagonal carbon planes (or graphene planes) in one domain and the orientation in neighboring graphite domains are not too mis-matched from each other so that these domains can be readily merged together when heated to a temperature above 2,000° C. (more typically above 2,500° C.). Such a heat treatment is commonly referred to as graphitization. Thus, the soft carbon can be defined as a carbonaceous material that can be graphitized. In contrast, a “hard carbon” can be defined as a carbonaceous material that contain highly mis-oriented graphite domains that cannot be thermally merged together to obtain larger domains; i.e. the hard carbon cannot be graphitized. Both hard carbon and soft carbon contain graphite domains that can be intercalated and thermally exfoliated. The exfoliated carbon then can be recompressed to produce a cathode layer having constituent graphite flakes being aligned.

The spacing between constituent graphene planes of a graphite crystallite in a natural graphite, artificial graphite, and other graphitic carbon materials in the above list can be expanded (i.e. the d₀₀₂ spacing being increased from the original range of 0.27-0.42 nm to the range of 0.42-2.0 nm) using several expansion treatment approaches, including oxidation, fluorination, chlorination, bromination, iodization, nitrogenation, intercalation, combined oxidation-intercalation, combined fluorination-intercalation, combined chlorination-intercalation, combined bromination-intercalation, combined iodization-intercalation, or combined nitrogenation-intercalation of the graphite or carbon material.

More specifically, due to the van der Waals forces holding the parallel graphene planes together being relatively weak, graphite can be treated so that the spacing between the graphene planes can be increased to provide a marked expansion in the c-axis direction. This results in a graphite material having an expanded spacing, but the laminar character of the hexagonal carbon layers is substantially retained. The inter-planar spacing (also referred to as inter-graphene spacing) of graphite crystallites can be increased (expanded) via several approaches, including oxidation, fluorination, and/or intercalation of graphite. This is schematically illustrated in FIG. 1(D). The presence of an intercalant, oxygen-containing group, or fluorine-containing group serves to increase the spacing between two graphene planes in a graphite crystallite and weaken the van der Waals forces between graphene planes, enabling easier thermal exfoliation.

The inter-planar spaces between certain graphene planes may be significantly increased (actually, exfoliated) if the graphite/carbon material having expanded d spacing is exposed to a thermal shock (e.g. by rapidly placing this carbon material in a furnace pre-set at a temperature of typically 800-2,500° C.) without constraint (i.e. being allowed to freely increase volume). Under these conditions, the thermally exfoliated graphite/carbon material appears like worms, wherein each graphite worm is composed of many graphite flakes remaining interconnected (please see FIG. 1(C)). However, these graphite flakes have inter-flake pores typically in the pore size range of 20 nm to 10 μm.

In one process, graphite materials having an expanded inter-planar spacing are obtained by intercalating natural graphite particles with a strong acid and/or an oxidizing agent to obtain a graphite intercalation compound (GIC) or graphite oxide (GO), as illustrated in FIG. 1(A). The presence of chemical species or functional groups in the interstitial spaces between graphene planes serves to increase the inter-graphene spacing, d₀₀₂, as determined by X-ray diffraction, thereby significantly reducing the van der Waals forces that otherwise hold graphene planes together along the c-axis direction. The GIC or GO is most often produced by immersing natural graphite powder (100 in FIG. 1(A)) in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium perchlorate). The resulting GIC (102) is actually some type of graphite oxide (GO) particles if an oxidizing agent is present during the intercalation procedure. This GIC or GO is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion, which contains discrete and visually discernible graphite oxide particles dispersed in water.

Water may be removed from the suspension to obtain “expandable graphite,” which is essentially a mass of dried GIC or dried graphite oxide particles. The inter-graphene spacing, d₀₀₂, in the dried GIC or graphite oxide particles is typically in the range of 0.42-2.0 nm, more typically in the range of 0.5-1.2 nm. It may be noted than the “expandable graphite” is not “expanded graphite” (to be further explained later). Graphite oxide can have an oxygen content of 2%-50% by weight, more typically 20%-40% by weight.

Upon exposure of expandable graphite to a temperature in the range of typically 800-2,500° C. (more typically 900-1,050° C.) for approximately 30 seconds to 2 minutes, the GIC undergoes a rapid volume expansion by a factor of 30-300 to form “exfoliated graphite” or “graphite worms” (104). Graphite worms are each a collection of exfoliated, but largely un-separated graphite flakes that remain interconnected (FIGS. 1(B) and 1(C)). In exfoliated graphite, individual graphite flakes (each containing 1 to several hundred of graphene planes stacked together) are highly spaced from one another, having a spacing of typically 2.0 nm-10 μm. However, they remain physically interconnected, forming an accordion or worm-like structure.

Exfoliated graphite worms can be mechanically compressed to obtain “recompressed exfoliated graphite” for the purpose of densifying the mass of exfoliated graphite worms, reducing inter-flake pore sizes or spaces, and aligning the orientation of the constituent flakes. (In some engineering applications, the graphite worms are extremely heavily compressed to form flexible graphite sheets or foils 106 that typically have a thickness in the range of 0.1 mm-0.5 mm.) In the instant invention, as illustrated in the lower right portion of FIG. 1(D), exfoliated graphite worms are compressed to the extent that the constituent graphite flakes are more or less parallel to one another and the edges of these flakes define an edge plane of the resulting block or layer of re-compressed graphite worms. Primary surfaces of some of the graphite flakes (top or bottom surfaces) can constitute a flake surface plane (as opposed to the edge plane).

Alternatively, in graphite industry, one may choose to use a low-intensity air mill or shearing machine to simply break up the graphite worms for the purpose of producing the so-called “expanded graphite” flakes (108) which contain mostly graphite flakes or platelets thicker than 100 nm (hence, not a nano material by definition). It is clear that the “expanded graphite” is not “expandable graphite” and is not “exfoliated graphite worm” either. Rather, the “expandable graphite” can be thermally exfoliated to obtain “graphite worms,” which, in turn, can be subjected to mechanical shearing to break up the otherwise interconnected graphite flakes to obtain “expanded graphite” flakes. Expanded graphite flakes typically have the same or similar inter-planar spacing (typically 0.335-0.36 nm) of their original graphite. Expanded graphite is not graphene either. Expanded graphite flakes have a thickness typically greater than 100 nm; in contrast, graphene sheets typically have a thickness smaller than 100 nm, more typically less than 10 nm, and most typically less than 3 nm (single layer graphene is 0.34 nm thick). In the present invention, expanded graphite flakes may also be compressed to form a layer of recompressed graphite having the desired orientation.

Further alternatively, the exfoliated graphite or graphite worms may be subjected to high-intensity mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill) to form separated single-layer and multi-layer graphene sheets (collectively called NGPs, 112), as disclosed in our U.S. application Ser. No. 10/858,814. Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm, but more typically less than 3 nm (commonly referred to as few-layer graphene). Multiple graphene sheets or platelets may be made into a sheet of NGP paper (114) using a paper-making process.

It may be noted that the “expandable graphite” or graphite with expanded inter-planar spacing may also be obtained by forming graphite fluoride (GF), instead of GO. Interaction of F₂ with graphite in a fluorine gas at high temperature leads to covalent graphite fluorides, from (CF)_(n) to (C₂F)_(n), while at low temperatures graphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In (CF)_(n) carbon atoms are sp3-hybridized and thus the fluorocarbon layers are corrugated consisting of trans-linked cyclohexane chairs. In (C₂F)_(n) only half of the C atoms are fluorinated and every pair of the adjacent carbon sheets are linked together by covalent C—C bonds. Systematic studies on the fluorination reaction showed that the resulting F/C ratio is largely dependent on the fluorination temperature, the partial pressure of the fluorine in the fluorinating gas, and physical characteristics of the graphite precursor, including the degree of graphitization, particle size, and specific surface area. In addition to fluorine (F₂), other fluorinating agents (e.g. mixtures of F₂ with Br₂, Cl₂, or I₂) may be used, although most of the available literature involves fluorination with F₂ gas, sometimes in presence of fluorides.

We have observed that lightly fluorinated graphite, C_(x)F (2≤x≤24), obtained from electrochemical fluorination, typically has an inter-graphene spacing (d₀₀₂) less than 0.37 nm, more typically <0.35 nm. Only when x in C_(x)F is less than 2 (i.e. 0.5≤x≤2) can one observe a d₀₀₂ spacing greater than 0.5 nm (in fluorinated graphite produced by a gaseous phase fluorination or chemical fluorination procedure). When x in C_(x)F is less than 1.33 (i.e. 0.5≤x<1.33) one can observe a d₀₀₂ spacing greater than 0.6 nm. This heavily fluorinated graphite is obtained by fluorination at a high temperature (>>200° C.) for a sufficiently long time, preferably under a pressure >1 atm, and more preferably >3 atm. For reasons remaining unclear, electrochemical fluorination of graphite leads to a product having a d spacing less than 0.4 nm even though the product C_(x)F has an x value from 1 to 2. It is possible that F atoms electrochemically introduced into graphite tend to reside in defects, such as grain boundaries, instead of between graphene planes and, consequently, do not act to expand the inter-graphene planar spacing.

The nitrogenation of graphite can be conducted by exposing a graphite oxide material to ammonia at high temperatures (200-400° C.). Nitrogenation may also be conducted at lower temperatures by a hydrothermal method; e.g. by sealing GO and ammonia in an autoclave and then increased the temperature to 150-250° C.

Compression or re-compression of exfoliated graphite worms or expanded graphite flakes into a layer or block of recompressed exfoliated graphite having a preferred graphite flake orientation can be accomplished by using several procedures, which can be classified into two broad categories: dry pressing/rolling or wet pressing/rolling. The dry process entails mechanically pressing graphite worms or expanded graphite flakes in one direction (uniaxial compression) without the presence of a liquid medium. Alternatively, as schematically illustrated in FIG. 1(E), the process includes feeding dry exfoliated graphite worms 30 into the gap between two counter-rotating rollers (e.g. 32 a and 32 b) to form a slightly compressed layer of “re-compressed exfoliated graphite,” which are then further compressed to form a thinner layer of further re-compressed exfoliated graphite (containing aligned graphite flakes) by directing the material into the gap between another two rollers (e.g. 34 a and 34 b). If necessary, another pair or multiple pairs of rollers (e.g. 36 a and 36 b) can be implemented to further reduce the layer thickness and further improve the degree of flake orientation, resulting in a layer 38 of relatively well-aligned recompressed exfoliated graphite.

A layer of oriented, recompressed exfoliated graphite structure (or multiple layers of such a structure stacked and/or bonded together) may be cut and slit to produce a desired number of pieces of the oriented, recompressed exfoliated graphite structure. Assuming that each piece is a cube or tetragon, each cube will then have 4 graphite flake edge planes and 2 flake surface planes as illustrated in the bottom right portion of FIG. 1(D). When such a piece is implemented as a cathode layer, the layer can be positioned and aligned in such a manner that one of the flake edge planes is substantially parallel to the anode layer or the porous separator layer. This flake edge plane typically is very close to or actually in direct contact with the separator layer. Such an orientation is found to be conducive to entry and exiting of ions into/from the electrode, leading to significantly improved high-rate capability and high power density.

It may be noted that the same procedures can be used to produce a wet layer of recompressed exfoliated graphite provided the starting material contains graphite worms dispersed in a liquid medium. This liquid medium may be simply water or solvent, which must be removed upon completion of the roll-pressing procedure. The liquid medium may be or may contain a resin binder that helps to bond together exfoliated graphite worms or flakes, although a resin binder is not required or desired. Alternatively and desirably, some amount of the liquid electrolyte (intended to become part of the electrolyte of the final multivalent metal-ion cell) may be mixed with the exfoliated graphite worms or expanded graphite flakes prior to being compressed or roll-pressed.

The present invention also provides a wet process for producing an electrolyte-impregnated recompressed graphite structure for use as a multivalent metal-ion battery cathode layer. In a preferred embodiment, the wet process (method) comprises: (a) preparing a dispersion or slurry having exfoliated graphite worms or expanded graphite flakes dispersed in a liquid or gel electrolyte; and (b) subjecting the suspension to a forced assembly procedure, forcing the exfoliated graphite worms or expanded graphite flakes to assemble into the electrolyte-impregnated re-compressed graphite structure, wherein electrolyte resides in the inter-flake spaces in recompressed exfoliated graphite. The graphite flakes of the exfoliated graphite or the expanded graphite flakes are substantially aligned along a desired direction. The recompressed graphite structure has a physical density from 0.5 to 1.7 g/cm³ (more typically 0.7-1.3 g/cm³) and a specific surface area from 20 to 1,500 m²/g, when measured in a dried state of the recompressed graphite structure with the electrolyte removed.

In some desired embodiments, the forced assembly procedure includes introducing an exfoliated graphite suspension, having an initial volume V₁, in a mold cavity cell and driving a piston into the mold cavity cell to reduce the suspension volume to a smaller value V₂, allowing excess electrolyte to flow out of the cavity cell (e.g. through holes of the mold cavity cell or of the piston) and aligning the multiple graphite flakes along a direction at an angle from approximately 45° to 90° relative to the movement direction of the piston. It may be noted that the electrolyte used in this suspension becomes portion of the electrolyte for the intended multivalent metal-ion cell.

FIG. 1(F) provides a schematic drawing to illustrate an example of a compressing and consolidating operation (using a mold cavity cell 302 equipped with a piston or ram 308) for forming a layer of highly compacted and oriented graphite flakes 314. Contained in the chamber (mold cavity cell 302) is a suspension (or slurry) that is composed of graphite flakes 304 randomly dispersed in a liquid or gel electrolyte 306. As the piston 308 is driven downward, the volume of the suspension is decreased by forcing excess liquid electrolyte to flow through minute channels 312 on a mold wall or through small channels 310 of the piston. These small channels can be present in any or all walls of the mold cavity and the channel sizes can be designed to permit permeation of the electrolyte species, but not the solid graphite flakes. The excess electrolyte is shown as 316 a and 316 b on the right diagram of FIG. 1(E). As a result of this compressing and consolidating operation, graphite flakes 314 are aligned parallel to the bottom plane or perpendicular to the layer thickness direction.

Shown in FIG. 1(G) is a schematic drawing to illustrate another example of a compressing and consolidating operation (using a mold cavity cell equipped with a piston or ram) for forming a layer of highly compacted and oriented graphite flakes 320. The piston is driven downward along the Y-direction. The graphite flakes are aligned on the X-Z plane and perpendicular to X-Y plane (along the Z- or thickness direction). This layer of oriented graphite flakes can be attached to a current collector (e.g. graphene mat) that is basically represented by the X-Y plane. In the resulting electrode, graphite flakes are aligned perpendicular to the current collector. Such an orientation is conducive to a faster ion intercalation into and out of the spaces between graphite flakes and, hence, leads to a higher power density as compared to the corresponding electrode featuring graphite flakes being aligned parallel to the current collector plane.

The configuration of a multivalent metal secondary battery is now discussed as follows:

A multivalent metal-ion battery includes a positive electrode (cathode), a negative electrode (anode), and an electrolyte typically including a metal salt and a solvent. The anode can be a thin foil or film of a multivalent metal or its alloy with another element(s); e.g. 0-10% by weight of Sn in Zn. The multivalent metal may be selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof. The anode can be composed of particles, fibers, wires, tubes, or discs of the multivalent metal or metal alloy that are packed and bonded together by a binder (preferably a conductive binder) to form an anode layer.

We have observed that a select multivalent metal (e.g. Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Mn, V, Co, Fe, Cd, Ga, or Cr), when coupled with a presently invented graphite or carbon material having expanded inter-graphene planar spaces, can exhibit a discharge curve plateau or average output voltage at approximately 1.0 volt or higher (up to 3.5 volts). This plateau regime of a discharge voltage vs. time (or capacity) curve enables the battery cell to provide a useful constant voltage output. A voltage output lower than 1 volt is generally considered as undesirable. The specific capacity corresponding to this plateau regime is typically from approximately 100 mAh/g (e.g. for Zr or Ta) to above 600 mAh/g (e.g. for Zn or Mg).

A desirable anode layer structure is composed of a network of electron-conducting pathways (e.g. mat of graphene sheets, carbon nano-fibers, or carbon-nanotubes) and a thin layer of the multivalent metal or alloy coating deposited on surfaces of this conductive network structure. Such an integrated nano-structure may be composed of electrically conductive nanometer-scaled filaments that are interconnected to form a porous network of electron-conducting paths comprising interconnected pores, wherein the filaments have a transverse dimension less than 500 nm. Such filaments may comprise an electrically conductive material selected from the group consisting of electro-spun nano fibers, vapor-grown carbon or graphite nano fibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaled graphene platelets, metal nano wires, and combinations thereof. Such a nano-structured, porous supporting material for the multivalent metal can significantly improve the metal deposition-dissolution kinetics at the anode, enabling high-rate capability of the resulting multivalent metal secondary cell.

Illustrated in FIG. 2(A) is a schematic of a multivalent metal secondary battery, wherein the anode layer is a thin multivalent metal coating or foil and the cathode active material layer contains a layer of thermally exfoliated graphite/carbon material that has been recompressed. Alternatively, FIG. 2(B) shows a schematic of a multivalent metal secondary battery cell wherein the cathode active material layer contains a layer of thermally exfoliated graphite/carbon material that has been recompressed, an optional conductive additive (not shown), and a resin binder (not shown) that helps to bond the worms together to form a cathode active layer of structural integrity.

The recompressed exfoliated graphite or carbon materials, when implemented as a cathode active material, enable the multivalent metal-ion cell to exhibit a voltage plateau portion in a discharge voltage-time or voltage-capacity curve obtained at a constant current density. This plateau portion typically occurs at a relatively high voltage value intrinsic to a given multivalent metal, and typically lasts a long time, giving rise to a high specific capacity. Typically, this plateau portion is followed by a slopping curve portion, corresponding to a supercapacitor-type behavior. The supercapacitor-type behavior (EDLC or redox) is due to the high specific surface area of the exfoliated graphite/carbon worms used in the cathode layer. In general, the plateau portion is increased and slopping curve portion decreased when the degree of re-compression of worms is increased.

The composition of the electrolyte, which functions as an ion-transporting medium for charge-discharge reaction, has a great effect on battery performance. To put multivalent metal secondary batteries to practical use, it is necessary to allow metal deposition-dissolution reaction to proceed smoothly and sufficiently even at relatively low temperature (e.g., room temperature).

In the invented multivalent metal-ion battery, the electrolyte typically contains a metal salt dissolved in a liquid solvent. The solvent can be water, organic liquid, ionic liquid, organic-ionic liquid mixture, etc. In certain desired embodiments, the metal salt may be selected from NiSO₄, ZnSO₄, MgSO₄, CaSO₄, BaSO₄, FeSO₄, MnSO₄, CoSO₄, VSO₄, TaSO₄, CrSO₄, CdSO₄, GaSO₄, Zr(SO₄)₂, Nb₂(SO₄)₃, La₂(SO₄)₃, MgCl₂, AlCl₃, Mg(ClO₄)₂, Mg(BF₄)₂, Grignard reagents, magnesium dibutyldiphenyl, Mg(BPh2Bu2)2, magnesium tributylphenyl Mg(BPhBu3)2), or a combination thereof.

The electrolyte may in general comprise at least a metal ion salt selected from a transition metal sulphate, transition metal phosphate, transition metal nitrate, transition metal acetate, transition metal carboxylate, transition metal chloride, transition metal bromide, transition metal nitride, transition metal perchlorate, transition metal hexafluorophosphate, transition metal borofluoride, transition metal hexafluoroarsenide, or a combination thereof.

In certain embodiments, the electrolyte comprises at least a metal ion salt selected from a metal sulphate, phosphate, nitrate, acetate, carboxylate, chloride, bromide, nitride, or perchlorate of zinc, aluminum, titanium, magnesium, calcium, manganese, cobalt, nickel, iron, vanadium, tantalum, gallium, chromium, cadmium, niobium, zirconium, lanthanum, or a combination thereof.

In the multivalent metal-ion battery, the electrolyte comprises an organic solvent selected from ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), methyl butyrate (MB), ethyl propionate, methyl propionate, propylene carbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), tetrahydrofuran (THF), toluene, xylene, methyl acetate (MA), or a combination thereof.

This invention is directed at the cathode active layer (positive electrode layer) containing a high-capacity cathode material for the multivalent metal secondary battery. The invention also provides such a battery based on an aqueous electrolyte, a non-aqueous electrolyte, a molten salt electrolyte, a polymer gel electrolyte (e.g. containing a metal salt, a liquid, and a polymer dissolved in the liquid), an ionic liquid electrolyte, or a combination thereof. The shape of a multivalent metal secondary battery can be cylindrical, square, button-like, etc. The present invention is not limited to any battery shape or configuration.

The following examples are used to illustrate some specific details about the best modes of practicing the instant invention and should not be construed as limiting the scope of the invention.

Example 1: Oxidation of Graphite and Thermal Exfoliation of Oxidized Graphite

Natural flake graphite, nominally sized at 45 μm, provided by Asbury Carbons (405 Old Main St., Asbury, N.J. 08802, USA) was milled to reduce the size to approximately 14 μm (Sample 1a). The chemicals used in the present study, including fuming nitric acid (>90% concentration), sulfuric acid (95-98%), potassium chlorate (98%), and hydrochloric acid (37%), were purchased from Sigma-Aldrich and used as received. Graphite oxide (GO) samples were prepared according to the following procedure:

Sample 1A: A reaction flask containing a magnetic stir bar was charged with sulfuric acid (176 mL) and nitric acid (90 mL) and cooled by immersion in an ice bath. The acid mixture was stirred and allowed to cool for 15 min, and graphite (10 g) was added under vigorous stirring to avoid agglomeration. After the graphite powder was well dispersed, potassium chlorate (110 g) was added slowly over 15 min to avoid sudden increases in temperature. The reaction flask was loosely capped to allow evolution of gas from the reaction mixture, which was stirred for 24 hours at room temperature. On completion of the reaction, the mixture was poured into 8 L of deionized water and filtered. The GO was re-dispersed and washed in a 5% solution of HCl to remove sulphate ions. The filtrate was tested intermittently with barium chloride to determine if sulphate ions are present. The HCl washing step was repeated until this test was negative. The GO was then washed repeatedly with deionized water until the pH of the filtrate was neutral. The GO slurry was spray-dried and stored in a vacuum oven at 60° C. until it was used.

Sample 1B: The same procedure as in Sample 1A was followed, but the reaction time was 48 hours.

Sample 1C: The same procedure as in Sample 1A was followed, but the reaction time was 96 hours.

X-ray diffraction studies showed that after a treatment of 24 hours, a significant proportion of graphite has been transformed into graphite oxide. The peak at 2θ=26.3 degrees, corresponding to an inter-planar spacing of 0.335 nm (3.35 Å) for pristine natural graphite was significantly reduced in intensity after a deep oxidation treatment for 24 hours and a peak typically near 2θ=9-14 degrees (depending upon degree of oxidation) appeared. In the present study, the curves for treatment times of 48 and 96 hours are essentially identical, showing that essentially all of the graphite crystals have been converted into graphite oxide with an inter-planar spacing of 6.5-7.5 Å (the 26.3 degree peak has totally disappeared and a peak of approximately at 2θ=11.75-13.7 degrees appeared).

Samples 1A, 1B, and 1C were then subjected to unconstrained thermal exfoliation (1,050° C. for 2 minutes) to obtain thermally exfoliated graphite worms. The graphite worms were compressed into layers of oriented, recompressed exfoliated graphite having physical density ranging from approximately 0.5 to 1.75 g/cm³, using both dry compression and wet compression procedures.

Example 2: Oxidation and Intercalation of Various Graphitic Carbon and Graphite Materials

Samples 2A, 2B, 2C, and 2D were prepared according to the same procedure used for Sample 1B, but the starting graphite materials were pieces of highly oriented pyrolytic graphite (HOPG), graphite fiber, graphitic carbon nano-fiber, and spheroidal graphite, respectively. Their final inter-planar spacing values are 6.6 Å, 7.3 Å, 7.3 Å, and 6.6 Å, respectively. Their un-treated counterparts are referred to as Sample 2a, 2b, 2c, and 2d, respectively. The treated samples were subsequently thermally exfoliated and recompressed to obtain samples of various controlled densities, specific surface areas, and degrees of orientation.

Example 3: Preparation of Graphite Oxide Using a Modified Hummers' Method and Subsequent Thermal Exfoliation

Graphite oxide (Sample 3A) was prepared by oxidation of natural graphite flakes (original size of 200 mesh, milled to approximately 15 μm, referred to as Sample 3a) with sulfuric acid, sodium nitrate, and potassium permanganate according to the method of Hummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. In this example, for every 1 gram of graphite, we used a mixture of 22 ml of concentrated sulfuric acid, 2.8 grams of potassium permanganate, and 0.5 grams of sodium nitrate. The graphite flakes were immersed in the mixture solution and the reaction time was approximately one hour at 35° C. It is important to caution that potassium permanganate should be gradually added to sulfuric acid in a well-controlled manner to avoid overheat and other safety issues. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The sample was then washed repeatedly with deionized water until the pH of the filtrate was approximately 5. The slurry was spray-dried and stored in a vacuum oven at 60° C. for 24 hours. The interlayer spacing of the resulting laminar graphite oxide was determined by the Debye-Scherrer X-ray technique to be approximately 0.73 nm (7.3 Å). Some of the powder was subsequently exfoliated in a furnace, pre-set at 950-1,100° C., for 2 minutes to obtain thermally exfoliated graphite worms. The graphite worms were recompressed using both the wet and dry press-rolling procedures to obtain oriented, recompressed exfoliated graphite worms.

Example 4: Oxidation and Thermal Exfoliation of Meso-Carbon Micro-Beads (MCMBs)

Graphite oxide (Sample 4A) was prepared by oxidation of meso-carbon micro-beads (MCMBs) according to the same procedure used in Example 3. MCMB microbeads (Sample 4a) were supplied by China Steel Chemical Co. This material has a density of about 2.24 g/cm³, an average particle size of 16 μm, and an inter-planar distance of about 0.336 nm. After deep oxidation treatment, the inter-planar spacing in the resulting graphite oxide micro-beads is approximately 0.76 nm. The treated MCMBs were then thermally exfoliated at 900° C. for 2 minutes to obtain exfoliated carbon, which also showed a worm-like appearance (herein referred to as “exfoliated carbon”, “carbon worms,” or “exfoliated carbon worms”). The carbon worms were then roll-pressed to different extents to obtain recompressed exfoliated carbon having different densities, specific surface areas, and degrees of orientation.

Example 5: Bromination and Fluorination of Carbon Fibers

Graphitized carbon fiber (Sample 5a), having an inter-planar spacing of 3.37 Å (0.337 nm) and a fiber diameter of 10 μm was first halogenated with a combination of bromine and iodine at temperatures ranging from 75° C. to 115° C. to form a bromine-iodine intercalation compound of graphite as an intermediate product. The intermediate product was then reacted with fluorine gas at temperatures ranging from 275° C. to 450° C. to form the CF_(y). The value of y in the CF_(y) samples was approximately 0.6-0.9. X-ray diffraction curves typically show the co-existence of two peaks corresponding to 0.59 nm and 0.88 nm, respectively. Sample 5A exhibits substantially 0.59 nm peak only and Sample 5B exhibits substantially 0.88 nm peak only. Some of powders were thermally exfoliated and then re-compressed to obtain oriented, recompressed exfoliated graphite.

Example 6: Fluorination and Thermal Exfoliation of Carbon Fibers

A CF_(0.68) sample obtained in EXAMPLE 5 was exposed at 250° C. and 1 atmosphere to vapors of 1,4-dibromo-2-butene (BrH₂C—CH═.CH—CH₂Br) for 3 hours. It was found that two-thirds of the fluorine was lost from the graphite fluoride sample. It is speculated that 1,4-dibromo-2-butene actively reacts with graphite fluoride, removing fluorine from the graphite fluoride and forming bonds to carbon atoms in the graphite lattice. The resulting product (Sample 6A) is mixed halogenated graphite, likely a combination of graphite fluoride and graphite bromide. Some of powders were thermally exfoliated to obtain exfoliated carbon fibers, which were then roll-pressed to obtain oriented, recompressed exfoliated graphite worms.

Example 7: Fluorination and Thermal Exfoliation of Graphite

Natural graphite flakes, a sieve size of 200 to 250 mesh, were heated in vacuum (under less than 10⁻² mmHg) for about 2 hours to remove the residual moisture contained in the graphite. Fluorine gas was introduced into a reactor and the reaction was allowed to proceed at 375° C. for 120 hours while maintaining the fluorine pressure at 200 mmHg. This was based on the procedure suggested by Watanabe, et al. disclosed in U.S. Pat. No. 4,139,474. The powder product obtained was black in color. The fluorine content of the product was measured as follows: The product was burnt according to the oxygen flask combustion method and the fluorine was absorbed into water as hydrogen fluoride. The amount of fluorine was determined by employing a fluorine ion electrode. From the result, we obtained a GF (Sample 7A) having an empirical formula (CF_(0.75))_(n). X-ray diffraction indicated a major (002) peak at 2θ=13.5 degrees, corresponding to an inter-planar spacing of 6.25 Å. Some of the graphite fluoride powder was thermally exfoliated to form graphite worms, which were then roll-pressed.

Sample 7B was obtained in a manner similar to that for Sample 7A, but at a reaction temperature of 640° C. for 5 hours. The chemical composition was determined to be (CF_(0.93))_(n). X-ray diffraction indicated a major (002) peak at 2θ=9.5 degrees, corresponding to an inter-planar spacing of 9.2 Å. Some of the graphite fluoride powder was thermally exfoliated to form graphite worms, which were then roll-pressed to produce recompressed exfoliated graphite material.

Example 8: Preparation and Testing of Various Multivalent Metal-Ion Cells

The exfoliated carbon/graphite materials prepared in Examples 1-7 were separately made into a cathode layer and incorporated into a multivalent metal secondary battery. Two types of multivalent metal anode were prepared. One was metal foil having a thickness from 20 μm to 300 μm. The other was metal thin coating deposited on surfaces of conductive nano-filaments (e.g. CNTs) or graphene sheets that form an integrated 3D network of electron-conducting pathways having pores and pore walls to accept a multivalent metal or its alloy. Either the metal foil itself or the integrated 3D nano-structure also serves as the anode current collector.

Cyclic voltammetry (CV) measurements were carried out using an Arbin electrochemical workstation at a typical scanning rate of 0.5-50 mV/s. In addition, the electrochemical performances of various cells were also evaluated by galvanostatic charge/discharge cycling at a current density from 50 mA/g to 10 A/g. For long-term cycling tests, multi-channel battery testers manufactured by LAND were used.

FIG. 3(A) shows the discharge curves of three Zn foil anode-based Zn-ion cells: first one containing a cathode layer of original graphite particles, second one containing a cathode layer of thermally exfoliated graphite (with only light recompression; pore size from 20 nm to 3 μm), and third one containing a cathode layer of thermally exfoliated graphite (with heavy recompression; pore size range from approximately 2 nm to 20 nm; having an edge plane being parallel to the separator and in contact with the separator). The electrolyte used was 1M of ZnSO₄ in water. These data indicate that the original graphite has very little ion storage capability; the non-zero, but minimal capability being likely associated with surface adsorption or electroplating of Zn on graphite particle surfaces (specific capacity <5 m²/g). In contrast, the layer of heavily recompressed worms, when properly oriented, appear to be capable of admitting and storing large amounts of Zn ions, possibly along with other ions dissociated from the electrolyte. The discharge curve exhibits a long plateau regime at 1.20-1.35 volts and a specific capacity as high as nearly 260 mAh/g. The resulting cell-level energy density is approximately 120 Wh/kg, very close to the energy densities of lithium-ion batteries. However, Zinc is more abundant, safer, and significantly less expensive than lithium. The layer of lightly recompressed worms, having somewhat randomly oriented graphite flakes (with respect to the separator plane) only delivers a moderate charge storage capability.

FIG. 3(B) shows discharge curves of two Ca-ion cells: one containing a cathode layer of thermally exfoliated graphite (with heavy recompression) having a flake edge plane being parallel to the separator and in ionic contact with the separator); the other having a graphite flake surface plane being parallel to the separator and in contact with the separator. The cell containing a cathode of original artificial graphite particles exhibits a very short discharge curve (<30 mAh/g); not shown in the chart. These data indicate that the presently invented cathode layer of oriented recompressed graphite worms exhibits an initial plateau regime corresponding to ion intercalation into the graphitic structure and a slopping curve portion corresponding to adsorption of Ca ions or other electrolyte-derived ions on graphite flake surfaces. The cell has a significantly higher specific capacity (250 mAh/g) as compared to the cathode featuring unfavorably oriented compressed worms (70 mAh/g). The latter is only capable of storing Ca ions via the surface adsorption or electroplating mechanism.

Summarized in Table 1 below are the typical plateau voltage ranges of the discharge curves of a broad array of multivalent metal-ion cells using natural graphite, artificial graphite, or graphite fiber having expanded d-spacing as a cathode active material. The specific capacity is typically from 100 to 650 mAh/g. In contrast, for each type of battery cell, the corresponding original graphite or carbon material does not enable any significant voltage plateau regime and does not provide any significant ion storage capability (typically <50 mAh/g).

TABLE 1 Plateau voltage ranges of the discharge curves in multivalent metal-ion cells. Anode Metal Voltage range Ba 3.40-3.55 V Ca 3.25-3.35 V La 2.91-3.05 V Mg 2.85-3.01 V Be 2.40-2.51 V Ti 2.18-2.22 V Zr 1.98-2.07 V Mn 1.78-1.85 V V 1.75-1.82 V Nb 1.67-1.73 V Zn 1.20-1.35 V Cr 1.16-1.31 V Ta 1.17-1.25 V Ga 1.09-1.18 V Fe 0.96-1.13 V Cd 0.95-1.10 V Co 0.87-0.98 V Ni 0.85-0.95 V

Shown in FIG. 4 are the discharge curves of two Ni mesh anode-based Ni-ion cells, one containing a cathode layer of original MCMB particles and the other a cathode layer of recompressed exfoliated MCMBs. The recompressed MCMB worms enable the MCMB to admit (via intercalation and then surface adsorption) and store a large amount of ions (up to 450 mAh/g). In contrast, the original MCMB beads with unexpanded inter-planar spacing stores a very limited amount of Ni ions, mostly due to electroplating at very low voltage levels. There is typically very short or no plateau regime in a charge or discharge curve for a multivalent metal-graphite/carbon cell having untreated, original carbon/graphite as the cathode active material.

The carbon or graphite material types, their respective inter-planar spacing values (prior to thermal exfoliation) and specific capacity values of the original graphite/carbon or exfoliated/recompressed worms when used as a cathode active material for Zn-ion, V-ion, and Mg-ion cells are summarized in Table 2 below:

TABLE 2 A list of carbon or graphite materials used as the cathode active material of an Al cell. Specific Specific Specific Sample Inter-planar capacity, capacity, capacity, No. Material spacing, Å mAh/g (Zn) mAh/g (V) mAh/g (Mg) 1a Natural graphite 3.35 22 25 1A GO, 24 hrs 5.5 223 211 1B GO, 48 hrs 7 301 287 1C GO, 96 hrs 7.6 346 269 2a HOPG 3.35 21 27 2A HOPG oxide 6.6 287 204 2b Graphite fiber 3.4 16 32 2B Oxidized GF 7.3 323 205 2c CNF 3.36 45 77 2C Oxidized CNF 7.3 332 277 3a Natural graphite 3.35 17 3A GO, Hummers 7.3 214 4a MCMB 3.36 22 4A Oxidized MCMB 7.6 261 5a Graphite fiber 3.4 16 5A CF_(0.9) 8.8 343 5B CF_(0.6) 5.9 204 6A CBrF_(x) 8.4 325 7A CF_(0.75) 5.85 202 7B CF_(0.93) 9.2 406

The following significant observations are made from Table 1 and related charts (FIG. 5-FIG. 7):

(1) In every group of carbon or graphite material used in the cathode of a multivalent metal-ion battery, the specific capacity of the cathode of exfoliated carbon/graphite materials are significantly higher than those of their original carbon/graphite counterparts.

(2) The present invention provides a powerful platform materials engineering technology for enhancing the specific capacity of carbon or graphite cathode materials implemented in a multivalent metal-ion battery.

(3) As demonstrated in FIG. 5, both the Zn-MCMB cell (containing a cathode layer of recompressed, exfoliated MCMB) and the Vanadium-CNF cell (containing a cathode of vapor grown carbon nanofibers that have been exfoliated and recompressed) show exceptionally stable cycle life, exhibiting less than 20% capacity degradation after 5,000 charge-discharge cycles. The cycle life of the presently invented multivalent metal-ion cell is typically significantly higher than the cycle life of the lithium-ion battery.

(5) FIG. 6 shows that the specific capacity of a Mg-ion cell containing a cathode layer of favorably oriented, heavily recompressed exfoliated MCMBs is significantly higher than the specific capacity of a Mg-ion cell containing a cathode of lightly recompressed exfoliated MCMBs. The edge plane orientation relative to the separator plane plays a critical role in dictating the charge storage capability.

(6) By supporting the multivalent metal (in a thin film or coating form) on a nano-structured network composed of interconnected carbon or graphite filaments (e.g. carbon nanotubes or graphene sheets) one can significantly increase the power density and high-rate capability of a metal-ion cell. This is illustrated in FIG. 7, which provides Ragone plots of four cells: a Mg-ion cell having a CNT-supported Mg anode and a cathode layer containing recompressed exfoliated artificial graphite, a Mg-ion cell having a Mg foil anode (no CNT support) and a cathode layer containing recompressed exfoliated artificial graphite, a Ca-ion cell having a graphene-supported Ca anode and a cathode layer containing recompressed exfoliated artificial graphite, and a Ca-ion cell having a Ca anode (no graphene support) and a cathode layer containing recompressed exfoliated artificial graphite. Both Mg-ion cells and Ca-ion cells deliver energy densities comparable to those of lithium-ion batteries, but higher power densities. The power density values are comparable to those of supercapacitors. In other words, the presently invented multivalent metal-ion batteries have the best of both worlds of lithium-ion batteries and supercapacitors.

This nano-structured network of interconnected carbon nano-fibers provides large surface areas to support multivalent metal and facilitate fast and uniform dissolution and deposition of metal cations at the anode side. This strategy also has overcome the passivating layer issue commonly associated with Mg or Ca metal anode. Other nano-filaments or nano-structures that can be used to make such a network include electro-spun nano fibers, vapor-grown carbon or graphite nano fibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaled graphene platelets, metal nano wires, or a combination thereof. 

We claim:
 1. A multivalent metal-ion battery comprising an anode, a cathode, a porous separator electronically separating said anode and said cathode, and an electrolyte in ionic contact with said anode and said cathode to support reversible deposition and dissolution of a multivalent metal, multivalent selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof, at said anode, wherein said anode contains said multivalent metal or its alloy as an anode active material and said cathode comprises a cathode layer of an exfoliated graphite or carbon material having inter-flake pores from 2 nm to 10 μm in pore size, the cathode layer comprises an active layer of recompressed exfoliated graphite or carbon material comprising a physical density of 0.5 to 1.8 g/cm³ and has a meso-scaled pores having a pore size of 2 nm to 50 nm.
 2. The multivalent metal-ion battery of claim 1, wherein said exfoliated carbon or graphite material in said cathode layer is selected from a thermally exfoliated product of meso-phase pitch, meso-phase carbon, meso carbon micro-beads (MCMB), coke particles, expanded graphite flakes, artificial graphite particles, natural graphite particles, highly oriented pyrolytic graphite, soft carbon particles, hard carbon particles, multi-walled carbon nanotubes, carbon nano-fibers, carbon fibers, graphite nano-fibers, graphite fibers, carbonized polymer fibers, or a combination thereof.
 3. The multivalent metal-ion battery of claim 1, wherein the active layer of recompressed exfoliated graphite or carbon material that is oriented in such a manner that said active layer has a graphite edge plane in direct contact with said electrolyte and facing or contacting said separator.
 4. The multivalent metal-ion battery of claim 3, wherein said active layer of recompressed exfoliated graphite or carbon material has a physical density from 1.1 to 1.8 g/cm³ and has pores having a pore size from 2 nm to 5 nm.
 5. The multivalent metal-ion battery of claim 3, wherein said active layer of recompressed exfoliated graphite or carbon material has a specific surface area from 20 m^(2/)g to 1,500 m^(2/)g.
 6. The multivalent metal-ion battery of claim 1, further comprising an anode current collector supporting said multivalent metal or its alloy or further comprising a cathode current collector supporting said cathode layer of exfoliated graphite or carbon material.
 7. The multivalent metal-ion battery of claim 6, wherein said anode current collector contains an integrated nano-structure of electrically conductive nanometer-scaled filaments that are interconnected to form a porous network of electron-conducting paths comprising interconnected pores, wherein said filaments have a transverse dimension less than 500 nm.
 8. The multivalent metal-ion battery of claim 7, wherein said filaments comprise an electrically conductive material selected from the group consisting of electro-spun nano fibers, vapor-grown carbon or graphite nano fibers, carbon or graphite whiskers, carbon nano-tubes, nano-scaled graphene platelets, metal nano wires, and combinations thereof.
 9. The multivalent metal-ion battery of claim 1, wherein said electrolyte is selected from an aqueous electrolyte, organic electrolyte, polymer electrolyte, molten salt electrolyte, ionic liquid electrolyte, or a combination thereof.
 10. The multivalent metal-ion battery of claim 1, wherein said electrolyte contains NiSO₄, ZnSO₄, MgSO₄, CaSO₄, BaSO₄, FeSO₄, MnSO₄, CoSO₄, VSO₄, TaSO₄, CrSO₄, CdSO₄, GaSO₄, Zr(SO₄)₂, Nb₂(SO₄)₃, La₂(SO₄)₃, BeCl₂, BaCl₂, MgCl₂, AlCl₃, Be(ClO₄)₂, Ca(ClO₄)₂, Mg(ClO₄)₂, Mg(BF₄)₂, Ca(BF₄)₂, Be(BF₄)₂, Alkyl Grignard reagents, tri(3,5-dimethylphenyl borane, tris(pentafluorophenyl)borane, magnesium dibutyldiphenyl, Mg(BPh2Bu2)2, magnesium tributylphenyl Mg(BPhBu3)2), or a combination thereof.
 11. The multivalent metal-ion battery of claim 1, wherein the electrolyte comprises at least a metal ion salt selected from a transition metal sulphate, transition metal phosphate, transition metal nitrate, transition metal acetate, transition metal carboxylate, transition metal chloride, transition metal bromide, transition metal nitride, transition metal perchlorate, transition metal hexafluorophosphate, transition metal borofluoride, transition metal hexafluoroarsenide, or a combination thereof.
 12. The multivalent metal-ion battery of claim 1, wherein the electrolyte comprises at least a metal ion salt selected from a metal sulphate, phosphate, nitrate, acetate, carboxylate, nitride, chloride, bromide, or perchlorate of zinc, aluminum, titanium, magnesium, calcium, beryllium, manganese, cobalt, nickel, iron, vanadium, tantalum, gallium, chromium, cadmium, niobium, zirconium, lanthanum, or a combination thereof.
 13. The multivalent metal-ion battery of claim 1, wherein the electrolyte comprises an organic solvent selected from ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), methyl butyrate (MB), ethyl propionate, methyl propionate, propylene carbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), tetrahydrofuran (THF), toluene, xylene, methyl acetate (MA), or a combination thereof.
 14. The multivalent metal-ion battery of claim 1, wherein the electrolyte also supports reversible intercalation and de-intercalation of ions at the cathode, wherein said ions include cations, anions, or both.
 15. The multivalent metal-ion battery of claim 1, wherein said cathode layer of exfoliated carbon or graphite material operates as a cathode current collector to collect electrons during a discharge of said multivalent metal-ion battery and wherein said battery contains no separate or additional cathode current collector.
 16. The multivalent metal-ion battery of claim 1, wherein said cathode layer of exfoliated carbon or graphite further comprises a binder material which bonds said exfoliated carbon or graphite material together to form a cathode electrode layer.
 17. The multivalent metal-ion battery of claim 16, wherein said binder material is an electrically conductive binder material selected from coal tar pitch, petroleum pitch, meso-phase pitch, a conducting polymer, a polymeric carbon, or a derivative thereof.
 18. The multivalent metal-ion battery of claim 1, wherein said battery has an average discharge voltage no less than 1.0 volt and a cathode specific capacity greater than 200 mAh/g based on a total cathode active layer weight.
 19. The multivalent metal-ion battery of claim 1, wherein said battery has an average discharge voltage no less than 1.0 volt and a cathode specific capacity greater than 300 mAh/g based on a total cathode active layer weight.
 20. The multivalent metal-ion battery of claim 1, wherein said battery has an average discharge voltage no less than 1.5 volts and a cathode specific capacity greater than 200 mAh/g based on a total cathode active layer weight.
 21. The multivalent metal-ion battery of claim 1, wherein said battery has an average discharge voltage no less than 1.5 volts and a cathode specific capacity greater than 300 mAh/g based on a total cathode active layer weight. 